1. Field of the Invention
The present invention relates generally to the field of disk drives and more particularly to transducers for heads thereof, the transducers including a pole tip protrusion (PTP) compensation layer to counteract the effect of pole tip protrusion.
2. Description of the Prior Art
Magnetic disk drives store and retrieve data for digital electronic apparatuses such as computers. A typical disk drive, as in FIGS. 1 and 2, comprises a head 100, including a slider 200 and a transducer 210, in very close proximity to a surface of a rotatable disk 110. As shown in a cross-sectional view of FIG. 3, the transducer 210, in turn, includes a write element 300 and optionally a read element 310. As the disk 110 rotates beneath the head 100, a very thin air bearing is formed between the surface of the disk 110 and an air bearing surface (ABS) of the slider 200. The air bearing causes the head 100 to “fly” above the surface of the disk 110. As the head 100 flies over the disk 110, the write element 300 and the read element 310 can be alternately employed to write and read data bits along data tracks on the disk 110.
In FIG. 3, the metallic components are shown with cross-hatching, while the dielectric components are shown as solid. In the write element 300, a coil 315 is wound around a yoke 330 transverse to the plane of the drawing. An electric current in the coil 315 induces a magnetic field in the yoke 330 that fringes outward around a write gap 340 to interact with the surface of the disk 110 in order to write bits to the tracks of the disk 110. It has been found that the heating the write element 300, for example by the process of writing, creates differential thermal expansion between the metallic components and the dielectric components that distorts the write element 300 and warps a terminus surface 320 of the transducer 210. The terminus surface 320 of the transducer 210 tends to be parallel with, and slightly recessed from, the ABS of the slider 200.
The effect of warping the terminus surface 320 is commonly referred to as pole tip protrusion (PTP) because the effect is most pronounced at a pole tip 350 of the write element 300. More specifically, the pole tip 350 protrudes towards the surface of the disk 110, effectively reducing the spacing between the write element 300 and the disk 110 (the “flying height”). The heating that leads to pole tip protrusion has a number of sources. When pole tip protrusion is caused by writing it is sometimes referred to as write-induced pole tip protrusion (WPTP), while pole tip protrusion caused by thermal changes (e.g., moving the drive into a warmer environment) is sometimes referred to as thermal pole tip protrusion (TPTP).
The flying height of the head 100 is a critical factor affecting the density of the data that can be stored on the disk 110. Accordingly, the magnetic recording industry has strived to increase the data storage density in both longitudinal and perpendicular recording technologies by employing various techniques aimed at decreasing the average flying height. One technique has been to employ write-induced pole tip protrusion.
Another technique for reducing the flying height of the head 100 is to incorporate a heating element into the slider 200 to temporarily heat a portion of the head 100 to cause the transducer elements 300, 310 to move closer to the disk, thereby controllably reducing the flying height during periods of reading and writing. This allows the flying height to be lower during reading and writing to enable higher data densities, and higher otherwise to enhance the durability of the head-disk interface. The technique of controllably reducing flying height when reading and writing is commonly known as dynamic flying height actuation.
Still referring to FIG. 3, heating elements 360, 370, and 380 are disposed in various locations within the write element 300. It will be appreciated that these elements may be operated in parallel or in series, may be fewer or greater in number, and placed at other locations in the transducer 210 than those shown. Examples of dynamic flying height heaters are provided in patent application Ser. No. 11/112,112, filed Apr. 22, 2005 and entitled “PERPENDICULAR MAGNETIC RECORDING HEAD WITH DYNAMIC FLYING HEIGHT HEATING ELEMENT.”
FIG. 4 models the effect of pole tip protrusion due to different heating sources. In FIG. 4 line 400 represents the terminus surface 320 as viewed in cross-section in the vicinity of the write gap 330 in the absence of any pole tip protrusion. Axis 410 is centered on the write gap 330. Line 420 represents the terminus surface 320 with the effect of thermal pole tip protrusion, line 430 shows the effect of write-induced pole tip protrusion, and line 440 shows the effect of pole tip protrusion induced by a dynamic flying height heater. To model thermal pole tip protrusion, the ambient temperature was set to 65° C. To model write-induced pole tip protrusion, an AC current was set to 35 mA with a frequency of 250 mHz. To model pole tip protrusion caused by a dynamic flying height heater, a model was built on a NiCr heater with a resistance of 60 ohm and operated at 150 mW.
A problem with dynamically reducing the flying height by causing the terminus surface 320 to bulge towards the disk 110 is that the point on the transducer 210 that is closest to the disk 110 is a point 390 because of the angled flight orientation of the head 100 as seen in FIG. 2. It has been found that pole tip protrusion also moves the point 390 even closer to the surface of the disk 110 than the write element 300. Thus, the point 390 strikes more asperities on the disk 110 and the probability of a catastrophic head crash increases.